This invention relates to a satellite transmission and receiving system in which audio and data information, among others, may be transmitted and received more efficiently and economically while complying with regulatory agency signal strength limitations.
Transmission of information such as audio, data, video, and the like via communication satellites has become commonplace in today's information technology society. In carrying out such transmission, it is desirable to use the strongest signal allowable so that the transmitted signal may be more accurately received by the least expensive equipment possible, i.e., using the smallest satellite receiving dish possible. On the other hand, the stronger the transmitted signal, the greater likelihood there is that such signal could interfere with other signals on the same satellite having closely related frequencies, signals located in nearby space but directed to or coming from other satellites, and signals transmitted from ground stations directly to other ground stations. In addition, most countries have established maximum acceptable satellite signal strengths for signals in given frequency bands. Oftentimes, these maximum limits make impossible or, at least, impractical, the use of smaller satellite receiving dishes since such dishes are not capable of accurately receiving signals whose strengths are under the allowed maxima. Of course, small satellite receiving dishes are desirable because they are easy to install and align, less aesthetically offensive, and are much lower in cost than larger more conventional satellite receiving dishes.
A number of modulation and transmission methods have been developed for transmitting signals via satellites some of which have attempted to overcome the signal strength limitation problem. The most commonly used method, however, known as the single carrier per channel (SCPC) method, generally does not overcome this problem and so the larger, more expensive satellite receiving dishes must be used with the method. In the SCPC method, the frequency bandwidth available for transmission of signals is divided into carriers, each having a bandwidth different from the bandwidths of the other carriers and each being assigned a "center frequency" located in the center of the carrier bandwidth. Each source of information such as audio information, data information, video information, etc., is considered a "single channel" and is modulated onto a respective one of the carriers, and each carrier carries only the information of its respective channel. Among the advantages of the SCPC method are the flexibility in the assignment of frequency and bandwidth and the allocation of power to each particular carrier, and generally lower power requirements. The major disadvantages of the SCPC method is the need for frequency stability in the reception of the transmitted signal and this generally requires the use of high stability (and high cost) crystals located in constant temperature ovens, and the use of phase-locked oscillators located on the satellite receiving dish. Also, as already mentioned, the limitations imposed on transmitted signal strength generally eliminates the use of small size satellite receiving dishes with the SCPC method.
One approach to overcoming the signal strength limitation problem is the so-called "spread spectrum" method. This method allows for the transmission of a very strong signal by moving the energy of the signal rapidly among different frequencies at the transmitting end. In this manner, the average signal strength is spread among a number of frequencies and therefore is maintained below the maximum allowed. However, this approach is expensive, requiring high cost receiving equipment, and only a few satellite carriers necessary with this method can be accommodated by a satellite transponder.
Another approach to overcoming the signal strength limitation problem is referred to as the FM-FM method and allows many carriers to share a common transmitter and satellite transponder. In particular, a number of frequency modulated carriers are combined into a common signal and then this common signal is modulated again onto a transmitted carrier. For example, a number of FM audio and frequency shift keyed (FSK) data signals may be multiplexed and modulated onto a wide band transmitted FM carrier. Among the disadvantages of this method is that all signals must originate from the same point since only one transmitter may be used and thus operators must deliver their audio or data information to the transmitter site; also, an entire satellite transponder or significant portion thereof, must be allocated for the method.